There is great need for storage of electricity, or of energy that can generate electricity. In some sense, fossil fuels are stored sunlight. Electro-chemical cells are good for storing energy for later retrieval, for example electro-chemical “batteries”, can be used for household items, small appliances and even for electric vehicles. But in size and cost, these are mostly small-scale applications.
But larger power applications cannot be adequately served with electro-chemical storage batteries. These use include storage of power for power plants or for other large machinery to be driven with stored energy. We refer to these as “large-scale” applications or “industrial” applications. In the electric power grid, hydroelectric power is often used to provide stored energy on-demand and enable fast load-following for regulation of grid variations. But these large-scale applications cannot be cost-effectively served with electrochemical storage and yet hydro-electric storage capacity is limited to regions and local conditions. Energy can also be stored as steam. There is a known industrial method for storing steam, done by storing pressurized boiling water in pipes and generating electricity by reducing the pressure, which causes the boiling point to drop and part of the stored steam to evaporate. But this causes a significant loss in free energy as the obtained steam has a lower temperature and pressure, and it is an expensive method with low thermal efficiency. But it works.
Pumped storage, which is similar to hydroelectricity, is widely practiced where the topology allows it but has only moderate efficiency. Compressed air can also be used but it only has moderate efficiency and relatively high costs. None are presently used to meet the large-scale demand in supporting the grid.
Furthermore, it will be understood that load requirements on the grid are large during the day and the early evening and lower at night and on the weekend. This heavy loading in the past was solved by designing power plants that could rapidly change power output without any significant loss in efficiency. In addition to hydroelectric power plants, steam power plants with boiler and turbine with large turndown ratio have that capability. They take time to start up (up to half a day), but they can operate at around 13% of their full capacity and still be at high efficiency and are able to vary their output rapidly for load-following or load-leveling on the grid. However, to be effective, this requires a large over-capacity compared to the average capacity of the plant. This overcapacity in our power system used to be around 1.5 to a factor of 2. But now power plants do not maintain such excess capacity.
The ratio between the maximum capacity and the minimum capacity at which the power plant can operate without a significant loss in efficiency is called the turndown ratio. For conventional coal power plants the design specifications call for a minimum turndown ratio of 8:1. These plants used to be effective providers and regulators of power on the grid, having been provided with adequate over-capacity.
However, demand has grown, and yet new plants have not been built at a rate to keep up with this growth. Rather, excess capacity has simply become part of the regularly used capacity. Furthermore, while today coal power plants are more efficient than fifty years ago, still overall they have become more expensive to operate per kWh, as the need to reduce pollution has increased their cost dramatically.
With this loss of overcapacity come the need to find a source of excess energy to meet peak demand and for use in gird regulation.
Furthermore, at the same time that over-capacity of the grid has substantially decreased, the variability of grid operations has increased dramatically. This has created a severe supply and control crisis. Not only do we have new user technology, like air-conditioning that is more variable in its use and increases load considerably, but we now have new variable generation sources, such as wind turbine, solar cells, and concentrating solar power (CSP), that can cause dangerous swings on the grid.
While CSP power plants are large enough to make it economical and practical to be provided with large-scale storage, most notably molten salt, solar cells and wind cannot. One way to cope with this condition is to build more fast-responding steam power plants, to increase the over-capacity and with fast response for supplying and regulating the grid. But building new power plants is a long process and is very expensive. An alternative solution that can increase capacity and controllability would therefore be welcome.
Another reason for the increased need for storage is that the technology for generating electricity has changed. Nuclear power plants have a much slower response and a low turndown ratio. Combined cycle power plants (CCPP) also have a very low turndown ratio, but for natural gas they have a much higher efficiency (60% versus 37-45% for coal) than any other fossil based power plant and are therefore in use. CCPP technology is based on a high temperature gas turbine, the hot exhaust of which is fed to a boiler creating steam for a steam turbine. These plants provide a large fraction of the electric energy in the world and their use is growing fast (reaching over 20% of installed capacity in the U.S.). The problem is that gas turbines have a very low turndown ratio, losing efficiency very rapidly when power is below maximum. The only control is basically on-off, as they can be shutdown in an hour and started up in one or two hours. But they are not suitable for rapid load following for grid regulation and there is not enough overcapacity to enable such operation as a practical matter.
The same is true for Integrated Gasification Combined Cycle (IGCC) power plants, the only really coal power plants clean enough to be added to the grid today in the US. These are basically combined cycle power plants in which the gas is not natural gas but the product of a coal gasifier. One of IGCC's problems is that its turndown ratio is very low and so is its capability to rapidly load follow is low. Again the need for grid regulation goes unmet.
Therefore, it will be appreciated that there remains a felt need for new energy storage systems that can help address the above supply and control issues. The present invention solves one or more of the problems associated with prior heat storage systems and is directed to these and other uses for stored energy.